Metals have been mankind’s most essential materials for thousands of years; however, their use is affected by ecological and economical concerns. Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency. However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength– ductility trade-off1,2. Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization3–6. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection7–11, the concept is overturned. We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase12); and transformationinduced hardening (resulting from the reduced mechanical stability of the room-temperature phase13). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels14,15 and massive solid-solution strengthening of high-entropy alloys3. In our transformationinduced plasticity-assisted, dual-phase high-entropy alloy (TRIPDP- HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength. Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility. This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials16,17. This metastabilityengineering strategy should thus usefully guide design in the nearinfinite compositional space of high-entropy alloys.

Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off